I found this on ebay and although it doesnt do the calculations for you it would incredibly useful to see the amps your system is pulling at different volumes and another probably more accurate volt gauge. With a littel custom fabrication i think it could be a winner.

I just spent the weekend on a houseboat with Grant and crew. His meter provides some neat information but we should ask outselves what do we want to achieve? After hanging our and playing tunes for a while it was neat to see 100+ amps going to his batteries. I have a dedicated voltmeter on my accessory dedicated battery bank (golf carts). I find my digital voltmeter provides sufficient information in a very simple inexpensive package. Now if things were not working as intended I would need to dig into my toolbox and grab my DC amp clamp, Grant's meter offers these capabilities on the fly. His system is also about 5x of my system in all regards (charging, batteries, and amplifiers), so his meter is a small component in his system. In my system it would be overkill. Between my perfect pass and this meter I have enough information to understand what is happening in my electrical system.